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8 significantly. It appeared that aggregate type, angularity, and variable of final asphalt viscosity (after mixing) and the inde- gradation had a greater effect on the energy required for com- pendent variables of aggregate temperatures and initial asphalt paction than did the compaction temperature. A difference in viscosity." However, they concluded that mixing temperature the type of polymer used also was found to be important, even was not a significant factor on asphalt hardening. The discus- though the binders met the same PG specifications. sions of the paper drew several criticisms of the analysis and In the field, however, temperature is considered one of the points of view disagreeing with the conclusions. primary factors to affect the compactability of HMA. Leiva Excessive temperatures during processing and storage of and West (18) found that field compactability of HMA was binders as well as during HMA production have several seri- dominated by mat temperature and mat thickness relative to ous consequences. Airey and Brown (28) reported that stor- mixture characteristics such as gradation, aggregate type, and age of binders at elevated temperatures can cause breakdown binder grade. Willoughby et al.(19), in a study of temperature of long chain polymers in modified asphalts, thereby negat- segregation during placement of HMA, found significant dif- ing the benefits of modified binders. Linde and Johansson ferences for in-place density resulting from localized tempera- also examined the effect of processing and storage tempera- ture differentials of only 14C (25F). In the field validation of ture on degradation of polymer modified binders (29). Tests the Pavecool program for estimating cooling rates of HMA were conducted with size exclusion chromatography (SEC) to during compaction, Chadbourn et al. (20) showed that almost detect changes in molecular sizes in the bitumen and the poly- no increase in mat density changes with continued passes of mer phase of the binders. Binders were stored at 200C (392F), rollers as the mat temperature dropped below 100C. Many in- and aliquots taken periodically for SEC testing and analysis. dustry references (2023) cite 79C (175F) as the cessation After just a few hours, the polymer degradation occurred as temperature for compaction of HMA in the field. However, it evidenced by a decrease in molecular size. The bitumen phase is probable that the cessation temperature limit is a general rule showed increases in molecular size most likely the result of of thumb and will vary from mix to mix depending to a large oxidation and polymerization reactions. However, in an inert degree on the grade or consistency of the binder. atmosphere, the polymer phase did not show changes in molecular size. The changes in molecular size were correlated to changes in mechanical properties of the binders. Tensile Effect of Temperature on Degradation properties dropped significantly for the binders showing of Asphalt Binders polymer degradation. A significant amount of research during the 1950s and 1960s Stroup-Gardiner and Lange demonstrated environmental also investigated asphalt aging. Some studies focused on the concerns associated with excessive HMA temperatures (30). hardening of asphalts that occurs during manufacturing of They reported greater volatile loss, emissions, and concentra- asphalt mixtures. In the 1958 AAPT symposium on asphalt tions of odor-causing compounds with increasing temperatures hardening, Clark (24) summarized that the mechanisms for a range of asphalt binders. They conducted a number of involved in age deterioration of asphalts are volatilization, oxi- studies on fumes and odors that can be released from asphalts dation, action of water, action of light, and chemical changes. and asphalt additives at different temperatures (31, 32). They He stated that volatilization was the primary cause of harden- used gas chromatography (GC) analyses to identify specific ing and that oxidation assumes a secondary role in aging of compounds that can be linked to nuisance odors. They also de- asphalt. Fink's paper (25) stated that "most oxidation reactions veloped a practical method of quantifying smoke and emis- approximately double for each 10C increase." Serafin (26) dis- sions potential from asphalt binders, called the SEP test. They cussed ways of minimizing binder hardening during the man- demonstrated that the release of volatile organic compounds ufacture of asphalt mixtures. The discussion touched on topics (VOCs), SEP mass loss rates, and opacity for asphalt binders are such as oven aging procedures, recovery procedures, tempera- strongly influenced by temperature and crude source. ture control charts, and penalties. Of interest for this research One of the most widely used references for asphalt around was how Michigan dealt with temperature control at plants. The the world is The Shell Bitumen Handbook (33). This handbook Michigan specification at the time required the "mixture be provides useful guidance on handling of asphalt binders. delivered at the temperature, between 275F and 375F, as Several specific recommendations are worth noting: directed by the Engineer, and shall not vary more than 20F, plus or minus, from that temperature, except that no mixture Bitumen should always be stored and handled at the lowest shall exceed a temperature of 375F." The discussion says that temperature possible, consistent with efficient use . . . to prevent auto-ignition of the bitumen 230C (446F) must never be ex- normal mix temperature was in the range of 285F to 325F. ceeded. . . . During mixing the hot bitumen must be readily able Lottman et al. (27) investigated the effect of mixing tem- to coat the dried and heated mineral aggregate, given the shear- perature on viscosity changes (hardening) of asphalt binders. ing conditions employed, in a relatively short period of time They found "a linear relationship between the dependent (typically 30 to 90 seconds); this determines the lowest mixing